The long-term objectives of the proposed research are to understand the dynamic stability characterizing the normal human microbiota and its beneficial activities. To achieve this goal requires a clear understanding of how these essential microbial communities form, how they function, and what maintains them in the face of natural and artificial perturbations. To help discover the rules underlying the complex interactions between the hundreds or thousands of microbial species present in and on the human body, scientists use simple model systems to provide a window into the fundamental principles by which different bacteria function together and with their host. One such model system is the light-organ symbiosis between a bioluminescent marine bacterium and its squid host. This highly specific symbiosis consists of only one bacterial species, but there are several distinct strains present in the symbiotic population in each animal. Understanding the rules by which these different strains interact will provide a foundation for understanding the way different species for healthy, stable communities in the human body. The specific aims of the proposed research are to (i) determine the natural trajectory the symbiotic population over the life of the host, (ii) determine the ways the population resists invasion by other bacteria, and (ii) determine how the symbiosis protects itself against 'cheaters', members that take resources from the rest of the population without contributing to symbiotic functions. These aims will be achieved by a combination of approaches including construction of bacterial mutants and following their colonization of the host by specific fluorescence imaging, raising juvenile hosts with specific symbiotic partners into adulthood, and modeling the population dynamics of the light organs by computational means. The results of this study will be made available to human microbiome researchers, who will be able to use the rules discovered here to better focus their studies of maintaining a healthy state in the complex sets of populations making up the microbial communities in the gut tract, the oral cavity, the skin and elsewhere on the body.